The present invention relates to electromechanical band-pass filters, i.e. filters whose resonant elements are cylindrical metal bars (resonators) interconnected by coupling members (couplers) vibrating in accordance with a selected mode and optionally having bridges over one or more resonators, and associated with electromechanical transducers. More particularly, the present invention concerns filters whose resonators vibrate longitudinally and couplers vibrate flexurally. Such filters are described in U.S. Pat. Nos. 4,163,960 for "Electromechanical filter structure" and 4,197,516 for "Bridged electromechanical filters", both assigned to the same Assignee as the present application.
The present invention has essentially for its object elemental cells of a filter whose band-pass reproduces with a high precision a value fixed beforehand, as well as to a filter obtained by arranging such cells in cascade. In practice, the problems for the filter manufacturer is to produce the most economic and often the most compact filter, whose transfer characteristic corresponds best to a given model. For this purpose, the manufacturer has the well known results of theoretical studies available to him and these mainly relate to electric filters and constitute the theory of filters. The results of the study of electric filters are then transposed to the mechanical field by applying an equivalent system between electrical and mechanical magnitudes well known in the art. Without going into detail on the theory of filters it is useful to point out the meaning of certain terms used hereinafter. The theory of filters reduces the study of the desired type filter (e.g. polynomial band pass) to that of a low-pass filter or so-called prototype filter. The basic electric cells forming the desired filter are obtained from a prototype filter by a simple mathematical conversion (corresponding to a change of the variable linked with the frequency). The band width of the cell forming the prototype filter is limited by the frequency beyond which the attenuation is no longer zero. By analogy, the band width of a filter cell is defined as the range of frequencies converted from the frequencies limiting the band of the prototype filter cell by the change of variable referred to hereinbefore. In the case of a complete filter obtained by arranging the cells in cascade form, said cells usually being of the same type, the response in the pass-band is defined by the type of approximation used for the design of the filter (Butterworth, Tschebycheff, etc) on the basis of the attenuation characteristic of the complete filter. The abutment for this is the model to be respected. This last condition makes it possible to define, taking account of the selected approximation type, the relationship of the impedances of the branches of the prototype filter cells and, due to the conversion of the variable, that of the cells of the desired filter. Electromechanical equivalents then make it possible to obtain characteristic magnitudes of the equivalent mechanical filter.
The article published by A. E. GUNTHER et al. in the Proceedings of the IEEE (January 1979, page 102) describes an example of a mechanical filter applied to a channel filter with a relatively broad pass band (6%) for a telephone transmission system and shows in FIG. 4 the circuit diagram of the cell of the electric filter corresponding to the mechanical filter shown in FIG. 3. Each cell of this filter has seven components. As described in this article, supplementary conditions are imposed by industrialization (identity of the bars, etc) leading to a compromise definition of the parameters of the mechanical elements. Despite the use of a relatively complex basic electric cell, the correspondence between the characterstic of the prototype filter cell and that of the produced mechanical filter cell no longer exists beyond a relative band width above the value of 6% defined in the article. This lack of correspondence results from the approximations made with a view to applying the equivalent system on a level with each of the branches of the cell. This error is unconstrained in the particular use described (relative band width of a few hundredths). However, in other cases, the difference between the real characteristic of the mechanical cell and the characteristic of the corresponding electric cell is large, making it necessary to adjust the mechanical filters produced to make them conform with the model. Such adjustments are difficult and onerous.